In radio frequency (RF) system applications, such as cellular and microwave communication system applications, where multiple carrier input signals are fed into an RF power amplifier, the amplifier's inherent nonlinear characteristics at higher power levels generally cause the amplifier output to contain undesirable intermodulation (IM) products. These IM products may cause undesirable interference and crosstalk over the operative amplifier frequency range. In addition, the IM products may exceed transmission standards for RF equipment.
One conventional method of reducing IM products uses a "spectral analysis" approach and is described in U.S. Pat. No. 4,879,519. The "spectral analysis" approach involves scanning the output of the amplifier using a receiver tuned to the frequency of candidate IM products. The IM level for a candidate IM product is read by the scanner, and a linearizer is adjusted to minimize the IM product. This procedure of reading the level of the IM product and adjusting to minimize the IM product is repeated until each of the IM products has a level below a predetermined acceptable level.
Although this conventional method reduces IM products, the method requires the scanner to iteratively search for the IM products and therefore has a slow convergence interval due to the time required to scan and find the IM products. In addition, this method requires sophisticated spectral analysis equipment which adds significant cost to the amplifier.
Accordingly, there is a need for a method and apparatus for amplifying multiple carrier input signals with a faster convergence time and reduced cost while still reducing IM products in an RF system.